It is desirable that a valve gear of an internal combustion engine has a function to change the valve opening timing of an engine valve and change the length of the valve open period of the engine valve to improve the combustion efficiency. In response to such a demand, Patent Document 1 discloses a technology for changing the length of the valve open period of an engine valve by changing one phase of double camshafts by phase changing means provided at the end portion of the camshaft. Further, Patent Document 2 discloses a technology for shifting the valve opening timing of an engine valve and changing the valve open period of the engine valve by changing the both phases of double camshafts by phase control mechanisms provided at the both ends of the camshafts.